The present invention relates to the digital image processing arts. More particularly, the present invention relates to a method for detecting halftones in the wavelet domain. The method is employed in a compression encoding procedure to trigger a descreening process (in the wavelet domain) whereby the halftone data are descreened prior to the compression process to increase compression. The method is also employed in a decompression decoding procedure to trigger the descreening process during image reconstruction to improve the quality of the reconstructed image.
The representation of digital image data in the wavelet domain is well known. Wavelet-based data compression schemes are also known and are expected to become increasingly popular. For example, JPEG 2000 is a newly developed wavelet-based data compression scheme that is expected to be widely implemented in the digital image processing arts.
It has been found that the presence of halftone image data in a digital image is disruptive to wavelet-based compression methods. Thus, halftone image data cannot be compressed as efficiently. Likewise, reconstruction of an image from compressed data that contains halftones results in a reconstructed image of sub-optimal quality.
In accordance with the present invention, an image processing method comprises receiving wavelet-domain data defining a digital image. The wavelet-domain data are processed in the wavelet domain to determine if halftone data are present in the wavelet-domain data. A descreening process, preferably a wavelet-domain process, is activated if halftone data are present in the wavelet domain data.
The method of detecting halftones in the wavelet domain includes determining an energy value for the sub-bands in a wavelet decomposition. The calculated energy values are compared to a model of energy values that are expected when halftone data are not present. If the actual energy values do not conform to the model, halftone data are deemed to be present. In one embodiment, the model defines a pattern of decaying sub-band energy for each sub-band moving from a highest level of the wavelet decomposition to the lowest level of the decomposition. In another embodiment, the expected energy values in the model vary depending upon the bit-rate of compression of the wavelet-domain data. If desired, violations of the model can be counted based upon the quantity and/or the magnitude thereof.
One advantage of the present invention resides in the provision of a method for detection of halftone image data in the wavelet domain.
Another advantage of the present invention is found in the provision of a method for detection of halftone image data in the wavelet domain wherein a wavelet-based halftone descreening method is triggered in response to detection of halftone data.
Another advantage of the present invention resides in the provision of a method for detection of halftone image data in the wavelet domain wherein the method is employed upstream and/or downstream from a wavelet-based image compressor to trigger descreening prior to compression or during image reconstruction.
A further advantage of the present invention results from the provision of a method for detecting halftones in the wavelet domain wherein halftone detection is based upon the absence of a preferred energy pattern in the wavelet sub-band data.
A yet further advantage of the present invention resides in the provision of a method for detecting halftone data in the wavelet domain wherein an expected energy pattern for the wavelet sub-band data is modeled and the actual energy pattern is compared to the model to determine if halftone data are present.
Still other benefits and advantages of the invention will become apparent to those of ordinary skill in the art to which the invention pertains upon reading and understanding this specification.